Most transmembrane proteins traverse the lipid bilayers with alpha- helices composed of 20-25 amino acid residues. The association of these alpha-helices in membranes is important to the biological functions of transmembrane proteins. In spite of this, little is known about the factors governing the helix-helix interaction and the energetics of association. This study will use small angle x-ray scattering to study the changes in association stability and energetics of glycophorin A transmembrane domain in respond to sequence variations. When the factors leading to association stability can be understood, predictions of structural features of transmembrane proteins will be advanced.